Harmonics Flashcards
The transmitted frequency is also called the ________
fundamental frequency
Pressure waves change shape as they propagate through tissue. Pressure wave changes shape with ______
depth
The classic tradeoff in ultrasound is penetration versus resolution. The use of harmonics somewhat lessens the tradeoff by allowing for receiving at a higher frequency, ______ and transmitting at the lower frequency _____
the 2nd harmonic frequency
the fundamental frequency
*not just 2nd harmonic energy is produced, but an entire spectrum of harmonics (2nd, 3rd, 4th, etc.). Currently ultrasound uses only the 2nd harmonic.
Traveling through the tissue causes the pressure wave’s_____________. This causes a vibration in the tissue, which produces a sound wave
Peak moves faster than trough = “lean forward”
With compression and rarefaction, the density of the medium changes, resulting in a change in propagation velocity. This change in propagation velocity is nonlinear and results in the generation of harmonic energy from the fundamental
The sound that comes back to the transducer (from the tissue) is _____ the fundamental frequency and called ________
double
the second harmonic
The harmonic frequencies are generated by the _____, not by the transducer
tissue
The waveform “leans” more as it travels. Therefore, the signal gets stronger with ______
depth
______ TDRs have the ability to transmit at the fundamental frequency and receive at the higher frequency requires broadband transducer capabilities.
broadband
Fundamental signal filtered out with_______ (or other) technology
pulse inversion
By transmitting multiple lines with different phases and then adding the resulting lines together, the fundamental energy adds destructively while the harmonic data adds constructively. As a result the harmonic energy gets stronger and there is no need to degrade the axial resolution to help eliminate the fundamental energy.
By using tissue Harmonic imaging, we get improved image quality due to:
- Primary beam is much narrower = improving lateral resolution
- Reduces grating lobes (an artifact)
- Harmonic beam is generated deep to the area where many artifacts occur (e.g., reverberation)
Since harmonic energy produced is dependent on incident pressure, the lower energy grating lobes produce much weaker harmonic signals, reducing the energy in the grating lobes.
Weaker grating lobes result in improved lateral resolution and less lateral translation of off-axis energy into the main beam.
Benefit and disadvantage of tissue Harmonics
benefits:
- reduction in grating lobes = improve lateral resolution
- reduction in “clutter” - Harmonic signals are significantly weaker than the fundamental signal in the nearfield. *one of the greatest advantages to harmonic imaging
- reduction in reverberation - weaker and narrower harmonic beam in the nearfield results in less reverberation artifact than occurs with the fundamental beam
Benefit and disadvantage of tissue Harmonics
disadvantage
Degradation in Axial Resolution
With harmonic imaging, a longer transmit pulse duration (PD) is usually used to reduce the bandwidth of the transmit signal. By reducing the transmit BW, there is less overlap between the transmit and receive bandwidth decreasing clutter in the image. However, the increase in PD also results in an increase in the SPL, decreasing the axial resolution.
Harmonics effects are best at ______
midfield
What is contrast Harmonics?
harmonic signals from microbubbles introduced into the body via a contrast agent
